This project involves several lines of work: (1) The role of anticodon loop pseudouridine in tRNA function. Mutants unable to form this modified base (hisT) are defective in regulation of several operons and show greatly diminished activity for several informational suppressors. The latter fact is being exploited to ascertain the biological importance of this modification. (2) Regulation of the histidine operon. This mechanism responds to the level of his-tRNA in the cell. This level is apparently perceived by the translation of a small peptide (16 amino acids) including seven adjacent his codons. We are pursuing a genetic analysis of the control region which includes the gene for this small peptide. (3) Work is continuing on regulation of proline degradative genes.